Conventionally known are various methods by which undetwisted portions and excessively detwisted portions are alternatingly formed in a multifilament yarn by means of false twisting. For example, Japanese Patent Publication Nos. 25065/75, 225/76 and 42662/76 disclose methods wherein a drawn multifilament yarn made of polyester fibers or polyamide fibers are false twisted by means of a spindle type false twisting device having a twisting peg therein at an excessively high temperature so that the fibers constituting the yarn are partially cohered to each other. Further in Japanese Patent Laid-open Nos. 143746/76 and 143749/76, and Japanese Patent Publication Nos. 15188/78 and 30818/78, methods are disclosed wherein a drawn multifilament yarn is false twisted by means of a false twisting device utilizing a turbulent fluid jet under a high overfeed.
In addition, methods are known in which a multifilament is positively subjected to a non-uniform treatment while it is being false twisted. For example, methods in which contacting conditions between a multifilament yarn and a heating device are varied are described in Japanese Patent Laid-open Nos. 66928/74, 15017/76 and 8119/77. Methods in which twists transmitted from a false twisting device toward a heating device are varied are described in Japanese Patent Publication No. 34016/76, and Japanese Patent Laid-open Nos. 554/74 and 121546/75. Japanese Patent Publication No. 8414/74, and Japanese Patent Laid-open Nos. 108353/74 and 61745/78 disclose methods in which the number of twists generated in a multifilament yarn by means of a false twisting device is varied. Methods in which the speed of a multifilament passing through a false twisting device is varied are described in Japanese Patent Laid-open Nos. 92337/74 and 92354/74. In Japanese Patent Laid-open Nos. 66722/77, 81749/78 and 101654/74, methods in which a multifilament yarn is irregularly false twisted along the length of the yarn are described.
All the above-described prior arts relate to a method for manufacturing an alternatingly twisted yarn by means of false twisting, however, they have a defect in that the average twist density over the entire yarn cannot be high because of the following reasons. (1) A large amount of non-twisted portions which are similar to those in a usual false twisted textured yarn are formed in addition to definite S-twist portions and Z-twist portions. (2) Relatively long non-twisted portions are formed between the S-twist portions and Z-twist portions. (3) The twist density in an undetwisted portion or an over detwisted portion is not uniform but is high at the center thereof and low at the ends thereof. (4) The twist densities in undetwisted portions or in over detwisted portions are varied. (5) The ratio of the length of undetwisted portions to the entire yarn length cannot be high. Furthermore, the alternatingly twisted yarns obtained in accordance with teachings described in the above-described prior arts have defects in that, although the yarn has twists in the same direction as that of the false twisting, it is slightly detwisted so that undetwisted portions are formed in which false twisted crimps appear and their compactness is lost; and that, although the yarn has twists in the opposite direction as that of the false twisting, the cohesion between the fibers constituting the yarn is so strong that over detwisted portions are formed in which the false twisted crimps do not appear and their bulkiness is lost, and accordingly in the yarns, difference between the configurations of the undetwisted portions and the over detwisted portions cannot be clearly distinguished from each other except by their twist directions.
It should be noted that the yarn of the present invention explained above cannot be manufactured easily in accordance with the above explained prior arts. More specifically, according to the above-explained prior arts, any alternatingly twisted yarn which is preferable as the object of the present invention cannot be obtained. In fact, some attempts have been made in order to eliminate the defects inherent in the above-explained prior arts, however, the attempts have not met the requirements. This is because, all the developments, which have been achieved are considered to be directed to improve the external factors with respect to formation of an alternatingly twisted yarn, e.g., the false twisting texturing conditions, such as the number of false twists, the tension in the yarn while it is treated or the heating temperature, the characteristics of the supply yarn which has to be textured, or the construction of the false twisting device but they are not directed to the mechanism itself by which an alternatingly twisted yarn is formed.
The present invention is based on research of the mechanism for forming an alternatingly twisted yarn because the mechanism is of importance.